The present invention relates to an optical pickup device which records or reproduces information on a recording medium by converging a light beam onto the recording medium.
It is desirable that an objective lens of an optical pickup device converges a light beam onto an information recording layer of a recording medium without aberration. Moreover, a decrease of the beam diameter of the light beam converged onto the information recording layer can increase the recording density of the recording medium and hence the recording capacity of the recording medium is increased.
One example of a method of decreasing the light beam increases the numerical aperture (NA). In a prior art, for example, the numerical aperture is 0.45 for a CD (compact disk) and 0.6 for a DVD (digital versatile disk. In recent years, the beam diameter is decreased by further increasing the numerical aperture.
However, for example, if the numerical aperture is increased to 0.85, it is difficult to form an objective lens by a single lens. Namely, the higher the numerical aperture, the more limited fabrication tolerance and assembling tolerance of the lens. Consequently, the objective lens formed by a single lens has difficulty in limiting the aberration within the tolerance and can not decrease the beam diameter.
Hence, if an objective lens and a spherical lens are combined to form an objective lens by two pieces of lenses, the fabrication tolerance and assembling tolerance of the lens are increased, thereby achieving a high numerical aperture.
Meanwhile, for the recording medium, the information recording layer is covered with a cover glass to provide protection against dust and scratches. More specifically, a light beam emitted from the objective lens passes through the cover glass, and converges on the information recording layer located under the cover glass and is hence focused. At this time, the beam diameter is minimized. However, if the thickness of the cover glass is not a predetermined value, the spherical aberration (SA) occurs and the beam diameter is increased. In this case, information can not be correctly read or written with respect to the information recording layer.
Moreover, the spherical aberration is proportional to the thickness error xcex94d in the cover glass and the fourth power of the numerical aperture NA.
SAxe2x88x9dxcex94dxc2x7NA4xe2x80x83xe2x80x83(1)
Therefore, even when the thickness error xcex94d is the same, the spherical aberration SA is increased with an increase of the numerical aperture NA. For instance, when the numerical aperture is 0.85, the spherical aberration SA is about four times larger than a spherical aberration when the numerical aperture is 0.6. Thus, as in the case of a numerical aperture of 0.85, when the numerical aperture is high, it is necessary to compensate for the spherical aberration caused by the thickness error in the cover glass.
On the other hand, Japanese laid-open publication (Tokukaihei) No. 8-212579 (published on Aug. 20, 1996) proposes a method of limiting an increase of the beam diameter by cancelling the spherical aberration caused by the thickness error in the cover glass and variations in the thickness of the objective lens.
In a conventional technique disclosed in the above publication, an objective lens unit 51 shown in FIG. 13 is provided. This objective lens unit 51 includes a first lens 52 and a second lens 53. The first lens 52 is held by a first holder 54. The second lens 53 is hemispherical and held by a second holder 55. Incidentally, the first holder 54 and the second holder 55 form a capacitor. Moreover, a cover glass 56b is provided on a side facing the objective lens unit 51 of a recording medium 56, and an information recording layer 56a is provided on the other side of the recording medium 56. Besides, a light beam 57 passed through the objective lens unit 51 is made to converge onto the information recording layer 56a. 
The electrostatic capacity C of the capacitor is given by
Cxe2x88x9dS/dxe2x80x83xe2x80x83(2)
wherein S is the area where the first holder 54 and the second holder 55 face each other and d is the distance between the first holder 54 and the second holder 55.
Thus, it is possible to control the distance d between the first holder 54 and the second holder 55 by detecting the electrostatic capacity C.
Moreover, in order to prevent a crash of the recording medium 56 and the objective lens unit 51 during a rotation of the recording medium 56, it is necessary to provide a work distance WD between the recording medium 56 and the objective lens unit 51. As shown in the paper xe2x80x9cHigh-numerical-aperture lens system for optical storage, Optics Letters, Vol. 18, No. 4, pp. 305-307, (1993)xe2x80x9d, the spherical aberration SA caused by this work distance WD is given by
SA=xe2x88x92(WD/8)xc2x7n2(n2xe2x88x921)sin4xcex80xe2x80x83xe2x80x83(3)
wherein n is the refractive index of the second lens 53 and sinxcex80 is the numerical aperture NA of the first lens 52.
The objective lens unit 51 formed by the first lens 52 and the second lens 53 is optically designed to eliminate the above-mentioned spherical aberration for a certain work distance WD. When a displacement from the above work distance WD is referred to as xcex94WD, a spherical aberration SA1 caused by this work distance xcex94WD is given by
SA1=xe2x88x92(xcex94WD/8)xc2x7n2(n2xe2x88x921)sin4xcex80xe2x80x83xe2x80x83(4)
according to equation (3).
Besides, as shown in the above-mentioned paper, a spherical aberration SA2 caused by the thickness error xcex94d in the cover glass 56b is given by
SA2=(xcex94d)2/(8a)n(nxe2x88x921)sin4xcex80xe2x80x83xe2x80x83(5)
wherein a is the radius of curvature of the spherical surface of the second lens 53.
Therefore, even if there are a thickness error xcex94d in the cover glass 56b and variations in the thickness of the objective lens unit 51, it is possible to cancel the occurrence of spherical aberration by cancelling out the spherical aberrations SA1 and SA2. In other words, the work distance WD needs to be controlled to an optimum value by changing the work distance WD according to the thickness error xcex94d in the cover glass 56b and the variations in the thickness of the objective lens unit 51.
In the above-mentioned conventional structure shown in FIG. 13, the distance between the first lens 52 and the recording medium 56 is adjusted to be uniform by a focusing operation. Furthermore, the distance between the first lens 52 and the second lens 53 is controlled to an optimum value by detecting the electrostatic capacity between the first holder 54 and the second holder 55. As a result, the work distance WD between the second holder 55 and the recording medium 56 is controlled to an optimum value. Consequently, even if there are a thickness error xcex94d in the cover glass 56b and variations in the thickness of the objective lens unit 51, it is possible to limit the occurrence of spherical aberration.
However, in the above-mentioned conventional structure, in order to detect the electrostatic capacity C of the capacitor formed by the first holder 54 and the second holder 55, it is necessary to lead conductors out of the first holder 54 and the second holder 55, respectively. On the other hand, in general, a focusing operation and a tracking operation for displacing the objective lens in the optical axis direction and a direction perpendicular to the optical axis are performed in an optical pickup. Therefore, the conductors viciously affect the performance of the focusing operation and the tracking operation of the objective lens unit 51. Namely, deterioration of the frequency characteristics and a tilt of the optical lens unit 51 occur. Moreover, since the conductors are long, a phase lag occurs in detecting the electrostatic capacity C due to the inductance of the conductors, etc., causing a problem that the frequency band for the detection can not be increased.
In order to solve the above problems, it is an object of the present invention to provide an optical pickup device capable of detecting the distance between lenses constituting an objective lens unit with high accuracy up to a high band and performing recording or reproduction of information on a recording medium accurately irrespective of an error in the thickness of the cover glass of the recording medium and variations in the thickness of the objective lens unit.
In order to achieve the above object, an optical pickup device of the present invention includes:
an objective lens unit including a first lens, a second lens for converging a light beam which passed through the first lens on a recording surface of a recording medium, and a lens distance adjusting section for adjusting the distance between the first lens and the second lens, wherein the second lens has a reflecting section for reflecting an outer part of the light beam which passed through the first lens and reached the second lens;
a reflected light beam detecting section for detecting a reflected light beam reflected by the reflecting section of the second lens; and
a control section for detecting the distance between the first lens and the second lens according to a result of detection by the reflected light beam detecting section and for controlling the lens distance adjusting section according to the result of detection.
According to this structure, the light beam incident on the objective lens unit passes through the first lens and reaches the second lens. The outer part of this light is reflected by the reflecting section of the second lens, while the center part of the light is made to converge on the recording surface of the recording medium by the second lens. Thus, writing or reading of information on the recording surface of the recording medium is performed by the center part of the light.
Meanwhile, the reflected light beam reflected by the reflecting section is emitted from the objective lens unit after passing through the first lens, and detected by the reflected light beam detecting section. The control section first detects the distance between the first lens and second lens of the objective lens unit according to the result of detection by the reflected light beam detecting section. Next, the control section controls the lens distance adjusting section according to the above result of detection.
Hence, according to this structure, since the distance between the first lens and the second lens is detected on the basis of the reflected light beam which passed through the first lens of the objective lens unit and was reflected by the second lens, there is no need to use conductors which are provided in a structure in which the distance between the lenses is detected on the basis of the electrostatic capacity between a holder for the first lens and a holder for the second lens. It is therefore possible to prevent the conductors from affecting the performance of the focusing operation and tracking operation of the objective lens unit and avoid a situation where the frequency band in detecting the distance between the lenses can not be increased.
Consequently, the distance between the lenses constituting the objective lens unit can be detected with high accuracy up to a high frequency band, and recording or reproduction of information on the recording medium can be performed accurately irrespective of an error in the thickness of the cover glass of the recording medium and variations in the thickness of the objective lens unit.